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排序方式: 共有48条查询结果,搜索用时 15 毫秒
1.
Macdonald S  Ashby J 《Nature》2011,471(7337):164-165
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2.
As one of his first acts upon becoming Astronomer Royal in 1835, George Airy made moves to set up a new observatory at Greenwich to study the Earth’s magnetic field. This paper uses Airy’s correspondence to argue that, while members of the reform movement in British science were putting pressure on the Royal Observatory to branch out into geomagnetism and meteorology, Airy established the magnetic observatory on his own initiative, ahead of Alexander von Humboldt’s request for British participation in the worldwide magnetic charting project that later became known as the ‘Magnetic Crusade’. That the Greenwich magnetic observatory did not become operational until 1839 was due to a series of incidental factors that provide a case study in the technical and political obstacles to be overcome in building a new government observatory. Airy attached less importance to meteorology than he did to geomagnetism. In 1840, he set up a full programme of meteorological observations at Greenwich – and thus turned his magnetic observatory into the ‘Magnetic and Meteorological department’ – only as the price of foiling an attempt by Edward Sabine and others in the London scientific elite to found a rival magnetic and meteorological observatory. Studying the origins of Airy’s Magnetic and Meteorological department highlights how important the context of other institutions and trends in science is to understanding the development of Britain’s national observatory.  相似文献   
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In a survey of the intracellular bile salt oxidoreductase activity in fecal bacteria, 16 strains of nuclear dehydrogenating clostridia and 2 strains of non-nuclear dehydrogenating C. paraputrificum were demonstrated unable to oxidize cholate at any of the 3 OH groups. Since nuclear dehydrogenation at the delta-1 and delta-4 position requires a 3-oxo precursor steroid, it appears that these organisms require the presence of a 3 alpha-hydroxysteroid dehydrogenating organism for nuclear dehydrogenation.  相似文献   
5.
I A Macdonald 《Experientia》1978,34(4):426-427
Tritium distribution on randomly labelled taurocholate (TC) was estimated at 28%, 4%, 1% and less than 0.5% on the hydrogens opposite the 3alpha-, 7alpha- and 12alpha-OH groups and taurine moiety respectively. Anomalously, C. perfringens 3alpha-hydroxysteroid dehydrogenase (3alpha-HSDH) catalyzed tritium loss of 36% on formation of 7alpha-, 12alpha-dihydroxy-3-keto-5beta-cholanoate, implying additional losses of tritium at other sites by this enzyme.  相似文献   
6.
The quantal contents of endplate potentials from extraocular muscles of an antarctic fish Pagothenia borchgrevinki were measured over a range of temperatures. Quantal release was maximal at about 5 degrees C but showed little dependence on temperature between -2 degrees C and 10 degrees C. Above 10 degrees C quantal content declined until release ceased about 18 degrees C. In view of the fact that the ambient temperature at which these fish live is constant at -1.9 degrees C, the results suggest that Pagothenia borchgrevinki is only partially adapted to its environment despite 25 million years acclimatization.  相似文献   
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D W Macdonald 《Nature》1992,360(6405):633-634
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9.
Johnston DT  Macdonald FA  Gill BC  Hoffman PF  Schrag DP 《Nature》2012,483(7389):320-323
Interpretations of major climatic and biological events in Earth history are, in large part, derived from the stable carbon isotope records of carbonate rocks and sedimentary organic matter. Neoproterozoic carbonate records contain unusual and large negative isotopic anomalies within long periods (10-100 million years) characterized by δ(13)C in carbonate (δ(13)C(carb)) enriched to more than +5 per mil. Classically, δ(13)C(carb) is interpreted as a metric of the relative fraction of carbon buried as organic matter in marine sediments, which can be linked to oxygen accumulation through the stoichiometry of primary production. If a change in the isotopic composition of marine dissolved inorganic carbon is responsible for these excursions, it is expected that records of δ(13)C(carb) and δ(13)C in organic carbon (δ(13)C(org)) will covary, offset by the fractionation imparted by primary production. The documentation of several Neoproterozoic δ(13)C(carb) excursions that are decoupled from δ(13)C(org), however, indicates that other mechanisms may account for these excursions. Here we present δ(13)C data from Mongolia, northwest Canada and Namibia that capture multiple large-amplitude (over 10 per mil) negative carbon isotope anomalies, and use these data in a new quantitative mixing model to examine the behaviour of the Neoproterozoic carbon cycle. We find that carbonate and organic carbon isotope data from Mongolia and Canada are tightly coupled through multiple δ(13)C(carb) excursions, quantitatively ruling out previously suggested alternative explanations, such as diagenesis or the presence and terminal oxidation of a large marine dissolved organic carbon reservoir. Our data from Namibia, which do not record isotopic covariance, can be explained by simple mixing with a detrital flux of organic matter. We thus interpret δ(13)C(carb) anomalies as recording a primary perturbation to the surface carbon cycle. This interpretation requires the revisiting of models linking drastic isotope excursions to deep ocean oxygenation and the opening of environments capable of supporting animals.  相似文献   
10.
ABSTRACT

Eighteenth-century events, replete with Dickensian dualities, brought two Enlightenment families to America. Pierre-Samuel du Pont and Joseph Priestley contemplated relocating their families decades before immigrating. After arriving, they discovered deficiencies in education and chemistry. Their experiences were indicative of the challenges in transmitting transatlantic chemistry. The Priestleys were primed to found an American chemical legacy. Science connected Priestley to British manufacturers, Continental chemists, and American statesmen. Priestley's marriage into the Wilkinson ironmaster dynasty, and Lunar Society membership, helped his sons apprentice, and befriend manufacturer-chemist Thomas Cooper. However, ideological persecution forced them from England. Priestley's plans for his sons to inherit Wilkinson's ironworks evaporated; in America, efforts to establish manufactories, colonies, farms, and a college miscarried. Cooper taught college chemistry, but his materialism provoked dismissals. The Du Ponts were unlikely founders of an industrial-chemistry empire. Du Pont's philosophy promulgated that agriculture, not industry, produced wealth. Eleuthère-Irénée apprenticed in France's gunpowder administration, however, plans for his succession died and director Antoine Lavoisier, a family friend, was executed. E.-I. and Du Pont's arrest precipitated relocation to America. Du Pont's utopian colony and schemes proved unrealistic. Nevertheless, E.-I.'s gunpowder manufactory—utilizing transatlantic contacts and privileged knowledge of advanced French chemistry—succeeded through practical application.  相似文献   
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